The present invention generally relates to a low noise converter of an outdoor unit for satellite broadcast reception use.
Generally, satellite broadcasting transmits electromagnetic waves in the range of 10.95 GHz through 12.75 GHz from a stationary satellite over the equator to the ground to effect a broadcasting operation to wider areas. A parabola antenna of high gain or a plane antenna thereof receives the waves and converts them into the signals of 1 GHz band near the antenna. This is because the received signals are required to be transmitted by a cable to an indoor receiver, such that the lower frequency allows the signal to be sufficient in strength. It is a low noise converter (LNB: Low Noise Block Downconverter) that affects the frequency converting operation.
The band width of the waves transmitted from the stationary satellite is normally 500 MHz through 800 MHz, in which the program of the satellite broadcasting of 1 through 24 channels are included. The low noise converter converts all the signals of this band width into the 1 GHz. For example, in the Europe Ku band, 11.7 GHz through 12.5 GHz are inputted, are converted into the signals of 950 MHz through 1750 MHz (800 MHz in band width) as IF signals, and are outputted so that the station can be selected by an indoor receiver.
FIG. 7 shows a circuit construction example of the conventional low noise converter. The input signals are amplified through a coaxial waveguide converting portion 1 by a low noise amplifier 2 of GaAs FET 2 through 3 step construction. Then, the received signals are passed into a band passing type filter 3 for preventing image signals or local signals so as to be mixed with an office transmitting signals from an office transmitting oscillator 5 by a mixer 4. They are further amplified by an IF amplifier 6 and are outputted as IF signals.
When the input signals are in the range of 11.7 GHz through 12.5 GHz, the office transmitting signals are 10.75 GHz and are converted into the frequencies of 950 MHz through 1750 MHz as IF signals. FIG. 8 shows a characteristic example of frequency relationship and a band pass type filter 3 utilized in the conventional low noise converter.
In the present invention, the frequencies which the satellite uses for the broadcasting operation are not only 11.7 GHz through 12.5 GHz (for example, the above-described Europe Ku band), but also frequency bands which are different from 10.95 GHz through 11.7 GHz (for example ECS (European Communication Satellite) band) and 12.25 GHz through 12.75 GHz (for example, Authert). Low noise converters adjusted for the frequency band are respectively prepared, so that a low noise converter is required to cooperate with the satellite to be received. Furthermore, a plurality of antennas and low noise converters are required when a plurality of satellites exist, the frequencies thereof are different respectively, or frequency bands are changed by the operation replacement of the satellites.
When the art of the circuit construction (which is designed to receive the signals of Europe Ku bands 11.7 GHz through 12.5 GHz) is extended so that signals in the range of 10.95 GHz through 12.5 GHz including, for example, the ECS band, are tried to be received, the conversion band width of the IF signal becomes 1550 MHz (=12.5 GHz through 10.95 GHz. Therefore, it is difficult for the indoor receiver to cover the range, thus considerably deteriorating the accuracy of the IF signals.
Also, in 2-band low noise converter (LNB: Low Noise Block Downconverter), so-called 2-band LNB for receiving two frequency bands, a signal for selecting a receiving band is required to be given to the 2-band LNB in order to receive two bands. Conventionally, a band switching input terminal 13, in addition to the IF output terminal 12 of the 2-band LNB1 as shown in FIG. 9, is provided. It is to be noted that in FIG. 9, reference numeral 14 is an input waveguide.
However, the 2-band LNB requires a sufficient airtight property for outdoor use thereof. It is an obstacle to maintain the airtight property to provide a band switching input terminal 13, in addition to an IF output terminal 12. This results in complicated construction.